As particulate controls for coal fired power plants have become more efficient, more waste is being contained within the plant and producing byproducts which require specialized disposal. In addition, the potential retrofit of older power plants with low-NO.sub.x burner technology so as to meet more stringent air emission regulations may result in byproducts having a higher carbon content. Fly ash is one of these byproducts which is increasingly becoming a disposal problem.
Much fly ash has a relatively high carbon content. For example, fly ash generated by the combustion of eastern bituminous coal in a pulverized coal-fired boiler retrofitted with low-NO.sub.x burners typically has a carbon content of greater than six percent. Although certain fly ash can be used as pozzolan and replace a portion of the cement in concrete, fly ash with a carbon content of greater than four percent cannot usually be sold for this purpose. As a result, there is often no beneficial use for fly ash having a relatively high carbon content. Furthermore, the outright disposal of such fly ash in landfills or otherwise is often complicated by its carbon content.
The particle size and heating value of fly ash make it difficult to beneficiate with conventional techniques which oxidize carbon therein so as to reduce the carbon content to acceptable levels. Direct combustion techniques, including reinjection of the fly ash into a power plant, have been examined and tested in the past. These techniques, however, have resulted in little or no further carbon reduction due to the insufficient residence time of the fly ash in the combustors. More specifically, the fly ash spends a valuable portion of its limited time in the combustor being heated to its autoignition temperature, the temperature at which the oxidation of the carbon in the fly ash accelerates greatly. As a result, the fly ash spends an inadequate amount of time in the combustor at a temperature at or above its autoignition temperature to reduce its carbon content to a desirable level.